Positive displacement pumps are characterized by alternately filling and emptying an enclosed volume by reciprocation of a piston in a cylinder, by the meshing of gears in an enclosed housing, or by the use of sliding veins, turning screws and other elements that can vary the volume of a pumping chamber. With a reciprocating piston, for example, the piston is moveable in an enclosed chamber in an intake stroke in which negative pressure is created within the chamber to draw fluid into the chamber and an exhaust stroke in which positive pressure is applied to the fluid in the chamber to expel it from the chamber. Such a reciprocating piston works in conjunction with valve structure which alternately respectively opens and closes inlets and outlets to the chamber so that the fluid flows in a desired direction through the pump. The pistons are often driven through drive linkages that include a series of shafts, rods and pins that move in bearings or guides. Examples of such linkages are found in gear, vein, screw and other positive displacement pumps.
One problem associated with positive displacement pumps of the past is that they typically produce vibration and noise, much as a result of the action of forces between and within the components of the linkages and pumping elements. Such noise is excessive for many applications, requiring substantial increases in volume, cost and energy consumption to muffle or suppress the vibrations or noise.
Another problem associated with positive displacement pumps of the past is that the interfaces between the components of the linkages and other surfaces require lubrication to reduce wear and to ensure smooth operation of the pump. Lubricants that are available to provide such lubrication often make their way into the fluid that is being pumped and thereby provide a source of contamination of the fluid. In air or other gas pumps where the mixture of the fluid with the lubricant becomes hazardous, or in chemical manufacturing or medical applications where the presence of the lubricant as a contaminant can ruin the material being produced or can present a danger to a patient being treated, the lubricants cannot be used to the extent needed, so the lubrication function is compromised.
A further problem with positive displacement pumps of the past has been their limited capability to dissipate heat generated by moving parts, particularly with high operating speeds, with the compression of gases taking place, and with metal-to-metal or other surface-to-surface contact taking place between linkage components and other surfaces. Such temperature can adversely affect the fluid being pumped and can result in damage to the pump.
An additional problem of positive displacement pumps of the past is the difficulty in restarting of the pump, particularly where outputs are pressurized. This problem has required the use of bleeder valves or other approaches which increase the complexity of the pump and the cost.
Attempts of the prior art to solve such problems have included the use of centrifugal pumps in which cylinders and pistons are rotated and the momentum of the pistons provides the force needed to move the pistons in a radially outward direction with mechanical linkages being provided for pulling the pistons radially inward. Such linkages are connected eccentrically to a stationary element so that each piston is alternately pulled radially inwardly and then allowed to move radially outwardly as the cylinder structure of the pump carrying the piston rotates about a central axis. Many centrifugal pumps have required excessive energy to operate and have been ineffective in reducing the noise problem.
Solutions to many of the above problems have been presented by the centrifugal pumps provided by Applicant as described in U.S. Pat. Nos. 4,990,062; 5,228,840 and 5,484,268, each owned by the assignee of the present application and each hereby expressly incorporated herein by reference. Such patents disclose centrifugal pumps including embodiments in which pistons are connected by linkages formed of non-elastic flexible elements connected to a fixed cam or roller to move the pistons inwardly as the unit rotates while allowing them to move outwardly by centrifugal force. Such pumps are particularly suitable for use in such devices as oxygen concentrators, particularly those for use in providing oxygen therapy to individual human patients. Oxygen concentrators which use such pumps are described and illustrated in the copending and commonly assigned U.S. patent application Ser. No. 08/745,281, filed Nov. 8, 1996, now U.S. Pat. No. 5,827,858 entitled "Rapid Cycle Pressure Swing Adsorption Oxygen Concentration Method and Apparatus", hereby expressly incorporated herein by reference.
Centrifugal pumps, however, rely upon the mass of the pistons or other rotating elements to develop the forces necessary to move the pumping elements radially outward. Such masses present design limitations on the pumps and on systems incorporating such pumps which can be undesirable for certain applications. Centrifugal pumps thus have been accompanied by certain disadvantages including the disadvantages of a higher than desirable weight and larger size than desirable in many applications.
Accordingly, there remains a need for a solution to the problems presented by positive displacement pumps of the prior art which do not possess the disadvantages of centrifugal pumps.